The present invention relates to improving the fatigue and stress resistance of transformation hardenable metal alloy components. More particularly, the invention relates to a process for improving the fatigue resistance cf turbine blades and other components with very high energy pulsed heat treatments.
Turbine blades are typically manufactured from high quality forged and cast alloys using conventional cutting or broaching operations. The performance of turbine blades is integrally related to both the design of the blade and the materials used to manufacture the blade. Turbine blades must withstand centrifugal forces due to the mass of the blade and the rotation of the rotor, bending stresses caused by axial and tangential forces exerted by hot gas or steam, vibrational stresses, and thermal stresses imparted by changes in temperature from ambient to operating temperatures. Many of these stresses concentrate at the blade root. For this reason, the mechanical and material requirements of turbine blades are quite high. Failure of a turbine blade can result in catastrophic loss of equipment, personal injury and potential loss of life.
Improving bending fatigue performance in cyclicly stressed parts often requires costly alternatives such as changing designs, specifying higher quality materials, or less often, specifying processes that can impact residual compressive surface stresses at those locations subject to fatigue failure. Shotpeening or particulate blasting techniques are examples of known processes that can locally induce compressive residual stresses by deforming or cold-working the surface of the part. Furnace hardening processes subject the entire part to unnecessarily high temperatures that can result in dimensional instability or in changes to desirable properties of the material.
A process of improving materials called carburizing is known in which carbon is introduced into the surface layer of a low carbon steel by heating a part in a furnace while it is in contact with a carbonaceous material. The carbon diffuses into the steel from the surface and converts the outer layer into high carbon steel. The part is then removed from the furnace, allowed to cool and heat treated at a temperature above the transformation point and quickly quenched. The high carbon surface layer is then transformed into a hard case containing martensite, while the low carbon core is left tough and shock resistant. The disadvantage of the carburizing process is that the part to be treated requires selective masking. Furthermore, the quenching step introduces distortion into the part, requiring a final grinding operation to correct the distortion. If carburizing were used to harden turbine blades the entire blade would need to be heated causing undesirable variations in the blades dimensions and changes in mechanical properties within the blade.
A process of improving parts by induction hardening is also known. In that prior art process, the part to be hardened is placed inside an induction coil. Rapidly alternating current flows through the coil quickly heating the portions of the part in contact with the coil. The depth of the heating is controlled by the frequency of the current. Conventional induction hardening also requires a quenching step which results in distortion in the treated workpiece. This distortion causes the necessity for final regrinding steps in high quality parts having critical tolerances.
Industrial lasers have been used to selectively harden portions of parts by inducing local martensitic phase transitions. Examples of such processes are found in United States Pat. Nos. 4,323,401, 4,533,400, and 4,617,070.
U.S. Pat. No. 4,304,978 discloses a transformation hardening process in which a laser beam is directed onto the surface of a transformation hardenable material at sufficiently high temperatures to produce an incandescent reaction with the workpiece. At the same time, the dwell time of the laser beam on the work surface is kept sufficiently short so that no significant melting of the workpiece takes place. However, this process has the disadvantage that a jet of cooling gas is required to quench the workpiece and prevent localized melting.
It is therefore an object of the present invention to provide a method for significantly improving the quality and fatigue performance cf turbine blades and other workpieces without treating the entire workpiece.
It is also an object of the present invention to provide a method for improving the fatigue resistance of turbine blades while producing only small local distortion, thus obviating the need for subsequent regrinding.
It is a further object of the present invention to provide a process for improving the fatigue resistance of turbine blades and other parts in nearly finish machined condition.
It is yet another object of the present invention to provide a process which can be adapted to numerous applications where locally superior metallurgical and fatigue resistance properties are desirable in workpieces of all types.